The present invention relates to a method and apparatus for forming an image on a light-sensitive material using an array of light-emitting diodes as point light sources. More particularly, the present invention relates to a method and apparatus for forming a halftone image on a light-sensitive material by controlling the contrast produced by the diode array.
A halftone image (latent image) can be formed on a light-sensitive material, such as a silver salt film, using a light source in the form of an array of tiny light-emitting diodes by controlling the intensity of the emitted light from individual diodes while the film is moved relative to the diode array in a direction perpendicular to the longitudinal direction of the diode array. The intensity of the light emission can be changed by controlling the electric current supplied to the individual light-emitting diodes used as point light sources. Since the light-emitting diodes have different current vs. intensity characteristics, the prior art apparatus for forming halftone images has employed a current supply circuit of the type shown in FIG. 7 or 8.
The circuit shown in FIG. 7 has a light-emitting diode array 1 connected between a power supply V.sub.DD and a ground terminal in such a way that a forward bias will be supplied from V.sub.DD to the individual LEDs. The cathode of each diode is connected to the ground terminal via a resistor and a transistor, and the intensities of light emission from the individual LEDs are controlled independently of one another by supplying the individual transistors with control signals S.sub.l -S.sub.n having levels corresponding to the contrasts to be produced. The resistivities of the resistors are adjusted in such a way that equal amounts of current will flow into the individual LEDs when control signals S.sub.l -S.sub.n of the same level are applied to the associated transistors. This is effective in providing the LEDs with uniform characteristics.
The circuit shown in FIG. 8 also has a light-emitting diode array 1 connected between a power supply V.sub.DD and a ground terminal in such a way that a forward bias will be supplied from V.sub.DD to the individual LEDs. However, transistors connected in series to the individual LEDs are connected to the ground terminal via a common resistor 2. This circuit configuration provides substantially uniform characteristics to the LEDs.
The conventional image-forming method employing the compensating means described above has had the following problems. The system shown in FIG. 7 is not suitable for high-resolution image forming, since it is extremely cumbersome to properly adjust the individual resistors when the number of LEDs employed is increased. In addition, the application of laser trimming and other techniques that are commonly used in IC fabrication technology is very difficult to implement in practical situations. The compensating system shown in FIG. 8 is a convenient method for producing high-resolution images, with a plurality of light-emitting diodes being arranged side by side. However, this arrangement is not well suited for providing precision in contrast, since the characteristics of the individual LEDs in the array cannot be rendered completely uniform.